AddThis has more than one billion users a month, and Facebook currently has 550 members.

However, the volume of sharing via the application doesn’t necessarily correspond to the number of user, so we know that it’s comparing apples to oranges to attempt to even suggest that 80 percent of Facebook members are using AddThis.

Most likely, a core group of frequent users share larger quantities of content via AddThis on Facebook. We’d love to know the size of this group.

While we’re at it, we’d love to see a break-out of which content gets shared most on Facebook in particular. Presumably AddThis could cull that from its already publicized rankings of most shared-from domains out of the more than seven million that have the application installed.

The data from AddThis does say something about the rivalry between Facebook and Google. The latter has only a third as many as the former sharing data using the orange icons, but the search engine’s is catching up with the social network.

It’s not really a surprise to see that AddThis sharing over Friendster and MySpace has fallen, since traffic to those sites has practically fallen off the side of a cliff.

As for the more popular venues for sharing content, what marketing possibilities do you see in the AddThis assessment shares on Facebook?